302 A TREATISE ON METAMORPHISM. 



Melanite is common in basic eruptive rocks rich in alkali. It occurs 

 especially with nephelite and leucite in phonolites, leucitophyres, nephe- 

 linites, and tephrites. In connection with contact metamorphism it occurs 

 with wollastonite and fassaite. It is also found in many serpentines. 



Uvarovite is at home in the serpentines, particularly those which con- 

 tain chromite. It is also found in the marbles. 



Common garnet, ordinarily a molecular mixture of two or more of the 

 species grossularite, pyrope, almandite, and melanite, is of course more 

 abundant than the pure species. It occurs in such rocks as amphibolites 

 and eclogites, in the metamorphosed diabases and gabbros, in the pyroxenic 

 rocks and their derivatives, and in the schists and gneisses both of igneous 

 and of sedimentary origin. 



Alterations. — The minerals into which garnets alter are very numerous, 

 chlorite (monoclinic, sp. gr. 2.71-2.725), talc (orthorhombic or monoclinic, 

 sp. gr. 2.75), and serpentine (monoclinic; sp. gr. 2.575), however, being the 

 more common products. Only the secondary products which occur on 

 an important scale in the rocks will be discussed, mere mineralogical 

 occurrences and pseudomorphs being ignored. 



Alterations of grossularite are not described in the standard text-books; 

 but it is known that meionite (tetragonal; sp. gr. 2.72) and zoisite (saus- 

 surite) (massive; sp. gr. 3.-3.04) are sometimes secondary products of garnet, 

 and it is natural to suppose that these minerals are derived either from 

 grossularite or from the grossularite molecule of common garnet, since 

 grossularite contains the elements in about the right proportions to produce 

 meionite and zoisite. 



Talc and serpentine are minerals which are secondary to garnet, and 

 from their chemical composition ought to be derived from the pyrope mole- 

 cule, either from the pure garnet or from the pyrope molecule in com- 

 bination with other garnet molecules. Pyrope is known to alter into 

 chlorite. As chlorite is regarded as a molecular mixture of serpentine and 

 amesite (crystallization not determined; sp. gr. 2.71), equations are written 

 for its alterations into amesite and into average chlorites. Pyrope further 

 alters into enstatite (orthorhombic; sp. gr. 3.2) and spinel (isometric; sp. gr. 

 3.8), these minerals frequently forming kelyphite rims about the garnet. 



Almandite is recorded as altering into chlorite and into hypersthene 

 (orthorhombic; sp. gr. 3 45) and spinel, which minerals form kelyphite rims 



